Enlargement of boreholes in carbonaceous strata for in situ combustion



3,076,506 ENLARGEMENT OF BQREHULES IN Chi-2B6- NACEGUS d'iRATA Ftlil EN SETS QGMEUQHQN Francis W. Crawford, liartlesviile, (Bria, assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Filed July 10, 1%), Ser. No. 826,136 8 Claims. (Cl. ltie ll) This invention relates to a process for enlarging boreholes in a permeable carbonaceous stratum and to the production of the stratum thru the resulting enlarged boreholes.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceous material is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to move thru the stratum by either inverse or direct air drive whereby the heat of combustion of a substantial propoltion of the hydrocarbon in the stratum drives out and usually upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition of carbonaceous material in a stratum around a borehole therein followed by injection of air thru the ignition borehole and recovery of product hydrocarbons and combustion gas thru another borehole in the stratum is a direct air drive process for effecting in situ combustion and recovery of hydrocarbons from the stratum. In this type of operation the stratum usually plugs in front of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this ditliculty and to permit the continued progress of the combustion zone thru the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustion zone from one or more surrounding boreholes.

ln situ combustion techniques are being applied to tar sands, shale, Athabasca sand and other strata in virgin state, to coal veins by fracturing, and to strata partially depicted by primary and even secondary and tertiary recovery methods.

To practice injection of a fluid into a bituminous rock reservoir, 21 hole is bored into the rock and the fluid pumped down the hole, across the interface between the rock and the hole, and then travels through the matrix of the rock. Major resistance to the flow occurs in the immediate vicinity of the borehole. In a flow system described by radial fiow theory, increasing the radius of the borehole increases the injection rate of fluid, other conditions remaining constant.

it is usually expensive to bore a large hole from the surface down to the bituminous rock, and so common practice is to bore the hole 4, 6, or 8 inches in diameter. Likewise, it is expensive to enlarge the diameter of the hole in the bituminous rock formation by the process of underrearning. Some of the benefits of a larger borehole can be obtained through the well known practices of acidization or fracturing. There are situations, however, in which these treatments would be unsatisfactory.

This invention is concerned with a simple and effective method of enlarging a well bore in an unconsolidated carbonaceous stratum.

Accordingly, the principal object of the invention is to provide a process for enlarging a well bore in sand or rock consolidated only by bituminous material. Another object is to enlarge a well in an unconsolidated carbonaceous stratum which avoids the necessity of drilling a Patented Feb. 5, 1&5

large hole from the surface down to the stratum. A further object of the invention is to provide a process for producing an unconsolidated carbonaceous stratum which requires lower injection pressures or faster injection rates with a given injection pressure. Other objects will become apparent upon consideration of the accompanying disclosure.

The method of the invention applies to strata in which the rock or sand particles are not cemented to each other by inorganic cementation, but rather are cemented by bituminous or normally solid hydrocarbon material. A broad aspect of the invention comprises contacting the wall of the well within the carbonaceous stratum with a liquid solvent for the bituminous material so as to dissolve the cementing material therefrom and free the rock or sand particles in the wall of the well so that the particles thus freed cave or fall into the well and then removing the freed rock or sand particles from the well. Solvents which are effective in dissolving the bituminous material include liquid hydrocarbons, such as diesel oil, kerosene, naphtha, gasoline, liquefied propane or butane, or other liquefied normally gaseous C to C hydrocarbons. it is desirable to at least double the well bore diameter within the stratum to be produced.

The solvent may be jetted against the wall of the well from a nozzle or spray, positioned on the end of a tubing string thru which the liquefied solvent is injected under pressure. The sand or rock is washed off the wall of the well as the bituminous material is dissolved and is periodically or continuously recovered by conventional means, such as pumping sand and liquid from the bottom of the hole. The spray or nozzle is raised and lowered along the axis of the borehole so as to enlarge the well from the top to the bottom of the stratum.

Another method of enlarging the borehole comprises injecting the solvent under pressure so as to force the some thru the wall of the well and into an annular secion of the stratum adjacent the well and flushing the solvent back and forth radially thru this annulus by alternately applying injection pressure on the solvent and pressurizing the stratum radially outside of the annulus by injecting pressurizing fluid, such as air, thru a ring of injection wells surrounding the well being enlarged. In this manner the bituminous material in a calculated annulus surrounding the borehole being enlarged is removed and the sand is freed to cave into the well from which it is then removed in any suitable manner as by means of a bucket or pump or by blowing out thru a. tubing string.

The enlarging technique of the invention is applied to the production well in an inverse air injection process wherein the central Well of a 5, 7, or 9-spot well pattern is ignited and utilized as a production well by injecting combustiomsupporting gas thru the surrounding ring of injection wells. In this manner the resistance to flow of gas thru the annulus around the production well wall is greatly reduced which proportionately decreases the injection pressure required for a given rate of production or movement of the production front.

It is also advantageous to enlarge the wells thru which the injected air is supplied prior to the ignition step. It is also advantageous to enlarge the central ignition borehole when propagating a combustion front from this well to the surrounding wells by direct drive wherein the combustion-supporting gas (air) is injected thru the central ignition borehole and the produced gases are recovered from the surrounding wells.

According to another embodiment of the invention, the wells in a line of ignition wells are each enlarged by the method of the invention and the stratum between the ignition wells is completely ignited so as to form a combustion front along the line of wells and this combustion front is then propagated thru the stratum to a line of injection boreholes generally parallel with the line of ignition boreholes so as to completely produce the stratum between the two lines of wells. When producing in this manner, it is also advantageous to enlarge the injection wells by the technique of the invention so as to further reduce the invention pressure required.

When producing a stratum by direct air injection, the sand is burned clean behind the combustion front and when operating in an unconsolidated sand this frees the sand to cave into the borehole. Hence, when operating in this manner it is desirable to consolidate the sand around the ignition and injection borehole by first burning the annulus around this borehole by inverse air injection utilizing a small concentration of fuel gas in the inversely injected air. After the combustion front has been moved by inverse air injection thru a foot or two of sand, reversal of the flow of air is effected by injecting the air thru the ignition borehole so as to drive the combustion front thru the stratum to the surrounding wells.

When initiating combustion around an ignition borehole in an unconsolidated sand after enlarging the same, utilizing inverse air injection technique with about 1 to 4 volume percent of fuel gas (such as propane) in the injected air, a carbonaceous cementing residue is laid down in the sand behind the combustion front which prevents further caving of the sand into the enlarged borehole. It is also feasible to consolidate the sand around a borehole after the enlarging step by chemical means such as injecting sodium silicate into the sand around the well and burning the carbonaceous material therefrom by direct drive whereby the sodium silicate serves as a cementing material in the unconsolidated burned out sand.

Certain modifications of the invention will become ap parent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

1 claim:

1. A process for producing hydrocarbons from an unconsolidated carbonaceous sand, cemented together solely by bituminous material, by in situ combustion which comprises removing a limited annulus of said sand around and adjacent an ignition well penetrating same so as to at least double the diameter of said ignition well within said sand by the steps of (1) dissolving bitumen from said annulus pressuring a liquid, non-aqueous solvent for said bitumen into said annulus and then driving said solvent back into said ignition well by injecting gas into the stratum radially outside of said annulus so as to carry dissolved bituminous material and sand into said ignition well; and (2) removing freed sand, solvent, and bitumen from said ignition well; thereafter igniting the bituminous material adjacent the resulting enlarged well bore to form a combustion zone therein; moving said combustion zone thru said sand radially from said ignition well toward an offset well therein by feeding combustion supporting, O -containing gas to said combustion zone whereby hydrocarbons are driven from said sand into one of the wells; and recovering produced hydrocarbons from said one of said wells.

2. The process of claim 1 wherein the steps of pressuring the solvent into the annulus and driving same back into the well are repeated and the driving step is effected by pressurizing said sand by injecting a fluid thru a ring of wells surrounding said ignition well.

3. The process of claim 1 wherein said o -containing gas is injected thru said ignition well and hydrocarbons are produced thru a ring of surrounding wells.

4. The process of claim 3 wherein the wells in said ring are enlarged by steps (1) and (2) prior to the ignition step.

5. The process of claim 1 wherein said o -containing gas is injected thru a ring of wells surrounding said ignition well and hydrocarbons are produced thru said ignition well.

6. The process of claim 5 wherein the wells in said ring are enlarged prior to the ignition step by steps (1) and (2).

7. The process of claim 1 wherein said solvent is a hydrocarbon.

8. A process for producing a permeable unconsolidated carbonaceous stratum which comprises enlarging each well of a line of ignition wells Within said stratum by removing a limited annulus of said stratum around each of said wells by steps (1) and (2) of claim 1; enlarging each well in a line of injection wells generally parallel to said ignition wells by said steps (1) and (2); igniting said stratum between said ignition wells to form a continuous combustion front along the line of ignition wells; thereafter injecting combustion supporting gas thru said injection wells so as to pass same to said combustion front and advance same toward said line of injection wells; and recovering the produced hydrocarbons thru said ignition wells.

References Cited in the file of this patent UNITED STATES PATENTS 1,473,348 Howard NOV. 6, 1923 2,139,595 Lerch et al Dec. 6, 1938 2,143,251 Savitz Jan. 10, 1939 2,288,857 Subkow July 7, 1942 2,745,647 Gilmore May 15, 1956 2,780,289 Garrison Feb. 5, 1957 2,888,987 Parker June 2, 1959 

1. A PROCESS FOR PRODUCING HYDROCARBONS FROM AN UNCONSOLIDATED CARBONACEOUS SAND, CEMENTED TOGETHER SOLELY BY BITUMINOUS MATERIAL, BY IN SITU COMBUSTION WHICH COMPRISES REMOVING A LIMITED ANNULUS OF SAID SAND AROUND AND ADJACENT AN IGNITION WELL PENETRATING SAME SO AS TO AT LEAST DOUBLE THE DIAMETER OF SAID IGNOTION WELL WITHIN SAID SAND BY THE STEPS OF (1) DISSOLVING BITUMENT FROM SAID ANNULUS PRESSURING A LIQUID, NON-AQUEOUS SOLVENT FOR SAID BITUMENT INTO SAID ANNULUS AND THEN DRIVING SAID SOLVENT BACK INTO SAID IGNITION WELL BY INJECTING GAS INTO THE STRATUM RADIALLY OUTSIDE OF SAID ANNULUS SO AS TO CARRY DISSOLVED BITUMINOUS MATERIAL AND SAND INTO SAID IGNITION WELL; AND (2) REMOVING FREED SAND, SOLVENT, AND BITUMEN FROM SAID IGNITION WELL; THEREAFTER IGNITING THE BITUMINOUS MATERIAL ADJACENT THE RESULTING ENLARGED WELL BORE TO FORM A COMBUSTION ZONE THEREIN; MOVING SAID COMBUSTION ZONE THRU SAID SAND RADIALLY FROM SAID IGNITION WELL TOWARD AN OFFSET WELL THEREIN BY FEEDING COMBUSTION SUPPORTING, O2-CONTAINNG GAS TO SAID COMBUSTION ZONE WHEREBY HYDROCARBONS ARE DRIVEN FROM SAID SAND INTO ONE OF THE WELLS; AND RECOVERING PRODUCED HYDROCARBONS FROM SAID ONE OF SAID WELLS. 